PhabricatorLiskDAO: Fragment serializer cache by class

Summary:
This restores the pre-PHP 8.1 behavior, where values of
static variables within inherited methods were independent
of each other. With PHP 8.1, this was changed to be truly
'static', which causes problems when one derivate of
PhabricatorLiskDAO defines a custom serializer but another
does not.

This came to light in T15726, but only for the Fund application,
which is a prototype, and deprecated. This fixes Fund, but
more importantly, everything else that would be broken by
this, whatever it was.

Ref: https://wiki.php.net/rfc/static_variable_inheritance

Previous stacktrace:

EXCEPTION: (RuntimeException) Undefined array key "totalAsCurrency" at [<arcanist>/src/error/PhutilErrorHandler.php:273]
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<phorge>/src/infrastructure/storage/lisk/PhabricatorLiskDAO.php:345]
  #1 <#2> PhabricatorLiskDAO::willWriteData(array) called at [<phorge>/src/infrastructure/storage/lisk/LiskDAO.php:1085]
  #2 <#2> LiskDAO::insertRecordIntoDatabase(string) called at [<phorge>/src/infrastructure/storage/lisk/LiskDAO.php:958]
  #3 <#2> LiskDAO::insert() called at [<phorge>/src/infrastructure/storage/lisk/LiskDAO.php:927]
  #4 <#2> LiskDAO::save() called at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:1405]
  [...]

Test Plan:
On PHP 8.1+:

1. Visit http://phorge.localhost/applications/ and enable the deprecated prototype applications "Fund" and "Phortune" via "Configure"
2. Visit http://phorge.localhost/phortune/merchant/edit/ and create a merchant
3. Visit http://phorge.localhost/fund/create/ and click the "Create New Initiative" button

Reviewers: O1 Blessed Committers, aklapper, valerio.bozzolan

Reviewed By: O1 Blessed Committers, aklapper, valerio.bozzolan

Subscribers: aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15064

Differential Revision: https://we.phorge.it/D25859